Collagen is one of the most successfully applied proteins, used, amongst other fields, in food industry as a film forming protein yielding flat or tubular edible and/or biodegradable films used as wrapping material in food packaging. Collagen is the generic term for a family of proteins, representatives of which can be found in any multicellular organism. To date, more than 20 different types of collagen have been described in the literature. For industrial purposes there are collagen sources which are particularly favorable for the recovery of collagen from the point of view of availability, tissue architecture and economy. One such source is bovine hide.
One of the most prominent current collagen-based technology is related with the manufacture of sausage casings based on collagen. This technology strongly tries to prevent the collagen fibers from losing their native molecular structure, since a substantial part of processing success and the mechanical properties of the resulting collagen tubular or flat films, depend on its fibrillar structure. Thus, a main objective of this technology is the preservation of the fibrous collagen structure, during all industrial steps of extracting and purifying the collagen from raw animal tissues, like bovine hide or porcine skin. Therefore, there exist in the state of the art carefully designed processes wherein, doughs of highly hydrated, basically intact acid swollen collagen fibers are obtained, that can be extruded into flat or tubular films. It is well known that the temperature control during collagen dough preparation and in extrusion is a very important factor to prevent the collagen fibers from hydrolyzing and, therefore, from gelatinizing.
Although acid swollen aqueous doughs of fibrous collagen have shown excellent properties in making films by extrusion at low temperatures, the huge dough viscosity, even at low solids (collagen) concentration, causes this material not to show rheological properties that would allow it to be readily processed by conventional extrusion equipment known from processing plastics. U.S. Pat. No. 3,123,482 and U.S. Pat. No. 3,346,402, for example, may give an impression on how complex and specialized extrusion equipment and conditions used in collagen casing manufacturing are.
Moreover, after passing through the orifice at the outlet of the extrusion head, the tubular or flat film formed from the collagen dough must be stabilized by coagulation and drying, what means that a large amount of water has to be removed from the shaped gel, resulting in high energy costs. Within such a process, collagen fibers are physically oriented and in many cases on-line or in a posterior chemical treatment crosslinked to yield a three dimensional matrix with a network structure of irreversibly embedded fibrils. This, on the other hand, presents the disadvantage that a product obtained by this kind of technology may not be recuperated and re-utilized in repeating the process. In addition, any other molded three-dimensional article different from those obtained by extrusion into films, tubes or strings are unfeasible, unless they are intended as soft hydrated goods, such as collagen soft fishing baits and lures, or optical lenses, since upon drying and in curing processes, said molded three-dimensional article shrinks dramatically until losing their original shape. Moreover, highly hydrated dispersions of fibrous collagen, although being useful in their classical forms of application, they do not have properties suitable to form three-dimensional solid articles by conventional thermoplastic processing techniques like heat extrusion and injection molding without running the risk of hydrothermally hydrolyzing the collagen to such an extent as to render very diluted gelatin solutions. As a result, solid collagen-based molded articles with a low level of water content have not yet been successfully achieved.
Thus, collagen-derived hydrogels, although capable to perform like thermoplastic materials, do not have the structure, stability or strength to perform like a solid molded article; even when they are dried, with the resulting expenditures, they will not perform stable under wet conditions.
Another collagen based technology, is characterized by the degradation of the collagen into gelatin, animal glues and hydrolyzates. Such hydrolysis products derived from collagen, differing in their degree of hydrolyzation, are used in a wide variety of industries as a food, as cosmetic ingredients and animal glues. According to this technology, collagen molecules are hydrolyzed, by enzymatic attack or heat treatment of an acidic or alkaline dispersion of fibrillar collagen in water, until the basic structural units are destroyed to yield a gelatin. The average molecular weight of these new peptic products is variable, and always below 500 kD. For practical purposes, the main characteristic of gelatin is its ability to generate physical hydrogels, even at very low solid concentration, which are thermo-reversible at temperatures around 40° C. This ability entails certain advantages in handling and molding processes since gelatin hydrogels can be heated over the melting point to be poured or injected into a mold, and next solidified by cooling. However, its gel toughness and strength is low and depends on the water content and on the degree of crosslinking. In the attempt of forming shaped articles, technical problems also arise from the extensive shrinkage that occurs during the drying of the hydrated product. Fabrication of molded articles consisting of gelatin or gelatin hydrolyzate with an average molecular weight of less than 3 kD is impossible.
Some new processes have been proposed to transform gelatin into a thermoplastic-like product for molding articles (see U.S. Pat. No. 4,992,100 and U.S. Pat. No. 5,316,717) by plasticizing a powdered gelatin, with the addition of water, into an extrusion device and submitting it to high temperatures and shear forces, to produce a flowable homogenous melt that, after drawing-off from the slotted die of the extruder, can be granulated. Granulates can be processed like a thermoplastic-like product, which presents the advantage of working at low water content and preserving some gelatin properties, whereas the shrinkage of the molded articles manufactured with said granulates is very slight and is settable by the addition of additives, for example, plasticizers. Nevertheless, gelatin is a product to be manufactured first from collagen, with the consequent production and acquisition cost. Moreover, its molecular structure entails a poor behavior upon aging of molded articles or under wet conditions, what has led to the consequence that in reality it has, to our knowledge, never found industrial application in molding articles.
In order to shorten the process of manufacturing gelatin and glue precursors (also called glutinous materials), another process has been proposed in German patent number DE 19712400. This process starts from a collagen-rich raw-material, and is capable of avoiding all those wet and thermo-chemical, time consuming, highly cost-operative and formerly required steps, by accomplishing the steps of: a) grinding the collagen raw-material to fibers; b) bringing this material to a moisture content of 5%-40%; c) subjecting the wet material for no longer than 60 min and under introducing heat to shear forces until the fibers lose their native three dimensional structure, to render an essentially homogeneous hydroplastic mass, with the main component having an average molecular weight of at least 500 kD and which is totally or partially soluble in water at above 45° C.; d) processing that hydroplastic mass to form a granulate, strands or sheets capable of being directly processed to gelatin or glues. Those materials are considered as precursors of gelatin and glues and they have a hard and brittle consistency, are somewhat soluble in warm water and are not able to form flexible films and articles which could substitute the performance of today's existing collagen films.
One important advance in the search for new industrial applications of collagen as a biodegradable and (in principle) edible material, would be to achieve a new technology for molding articles, starting from native collagen without requiring the prior manufacture of gelatin. Therefore, an important advancement in manufacturing collagen-based goods, such as extruded tubular or flat films and molded solid articles, would be the development of a process capable of avoiding the step of manufacturing a precursory plastic “wet” mass based on acid- or alkali-swollen aqueous dispersions of collagen fibers. That type of gel-like dispersions suffer from elevated production costs, low flowability (high viscosity), consequently difficult to process and the need of removing the water after molding and/or extrusion.
In view of the above stated there is the need in the art of an alternative collagen-based technology for the production of collagen-based goods of different shapes including flat or tubular films and three-dimensional articles of different shapes and sizes. Advantageously said technology should avoid, on one hand, the manufacture of gelatin intermediates associated with time consuming and high costs processes. On the other hand said technology should also avoid the disadvantages associated with the use of native collagen above stated, and the disadvantages associated with aqueous dispersions of collagen fibers which are difficult to process as above mentioned.
There is therefore the need in the state of the art of a new collagen-precursor which can be used in the preparation of a collagen-based composition performing like a thermoplastic.
There is also the need of an alternative collagen-based composition which may advantageously, be processed by conventional plastic techniques known in the art, and is suitable to be conformed into solid shaped articles which are edible and biodegradable. Moreover said articles are thermosealable, and show improved properties over collagen-based articles known from the state of the art such as, among other, resistance to water, tensile strength, minimal shrinkage of the conformed articles.